Fiber protection is herein defined to be the ability in a fiber network to have all traffic still functional in the case where a fiber segment is broken. Equipment protection is a higher degree of protection that is also more expensive and means that the traffic in the network is functional also in the case that part of the transmission equipment, e.g. lasers, receivers etc., is malfunctioning.
Fiber protection is used when the risk of a failure of any segment of the transmission fiber is relatively high compared to the risk that the rest of the transmission/reception equipment fails, and when a complete equipment protection is too expensive.
Examples of patents describing fiber protection in WDM ring networks are described in U.S. Pat. Nos. 5,680,235, 6,134,036 and 6,278,536, In the networks disclosed in these patents the fiber protection is carried out by switches or other on/off devices connected in the main ring fiber.
Protection in ring networks using communication on two parallel optical fibers to achieve bidirectional communication is disclosed in e.g. U.S. Pat. No. 5,510,917, and published International patent application No. WO 00/28670.
Furthermore, in a WDM ring network, both protected and unprotected channels can exist at the same time. The normal way of using unprotected traffic is to set up a path using equipment devoted only for these channels. If the path is broken, the traffic will be lost. In a protected network, each protected channel has two different paths reserved for it. One of the paths will be active and the other one inactive. In order to be able to send traffic in both paths, some equipment must be doubled. Examples of patents describing protection in combination with switches include the already cited U.S. Pat. Nos. 5,680,235 and 6,134,036, and also U.S. Pat. No. 5,933,258. In published European patent application No. 0 928 082 an example of low priority traffic is described that can be disabled by operating switches.
The concept “low priority channels” as used herein describes channels that may be disabled either by fiber breaks or decisions by switching devices. One kind of low priority channels is the conventional unprotected point-to-point channel. In ring networks, unprotected traffic will use only one of the two possible paths around the ring. The conventional unprotected channel will use its own add-drop filters to connect to the ring and it will be independent of other equipment.
In copending Swedish patent application No. 0101300-2, filed 11 Apr. 2001, and U.S. provisional patent application No. 60/288,422, filed 4 Apr. 2001, corresponding to published International patent application WO 02/084915 and U.S. patent application “Low loss WDM add drop node”, filed Nov. 12, 2002, which are all incorporated herein by reference, a WDM ring network is disclosed in which bidirectional traffic is carried on a single fiber.